Allele

From Wikipedia, the free encyclopedia

An allele (/əˈliːl/)[1][2] is a variant form of a given gene.[3] Sometimes, the presence of different alleles of the same gene can result in different observable phenotypic traits, such as different pigmentation. A notable example of this trait of color variation is Gregor Mendel's discovery that the white and purple flower colors in pea plants were the result of "pure line" traits which could be used as a control for future experiments. However, most genetic variations result in little or no observable variation.

Most multicellular organisms have two sets of chromosomes; that is, they are diploid. In this case the chromosomes can be paired: each pair is made up of two chromosomes of the same type, known as homologous chromosomes. If both alleles at a gene (or locus) on the homologous chromosomes are the same, they and the organism are homozygous with respect to that gene (or locus). If the alleles are different, they and the organism are heterozygous with respect to that gene.

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✪ Alleles and Genes

✪ What is an allele ? ( Allele examples )

✪ Alleles and genes

✪ Genes vs Alleles

✪ What is an ALLELE ?

Transcription

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I don’t remember which grade it was where
I learned something about my tastebuds that
can never be unlearned, but the event and
the lesson with genetics has stuck with me
forever.
For you see, I learned that my tastebuds cannot
taste PTC.
Let me preface this with explaining that PTC
stands for this –we’ll stick with PTC---and
it’s a chemical that can be sold on these
paper strips.
It can be purchased under the name PTC paper,
and it is popular in genetic classes because
it has this fascinating quality: some people
put it on their tongue and immediately say,
“Yuck, this is bitter!”
And some people, when they place the paper
on their tongue…taste absolutely nothing.
Well, unless you consider the paper.
Does paper have a taste in itself?
That’s a debatable question but the point
is…some people can taste PTC.
Some people cannot taste PTC.
And I was really disappointed, because I remember
that I was the only one there that could not
taste it so here was everyone getting this
amazing science experience and I couldn’t
taste a thing.
Well….
there may have been more than just me that
couldn’t taste it in the classroom that
day, but they didn’t seem as concerned by
the fear of missing out of the PTC paper as I
was.
I remember someone trying to make me feel better by saying, “Oh, but it tastes bitter!
You’re actually lucky.”
Then they tried to describe what it tasted
like to me.
But it’s not the same; I guess I’ll never know for myself what it would have
tasted like.
Of course, the reason PTC paper is used in
genetic classes is because the trait of being
able, or not being able, to taste PTC is based
on genetics!
A reminder from our intro to heredity unit
that genes are portions of DNA, and they have
the ability to code for a characteristic---
a trait.
Like being able to taste, or not to taste,
PTC.
Now we do want to point out that many traits
are actually coded for by interactions of
more than one gene.
Like eye color, which is quite complex, and
determined by interactions of many genes together.
In fact, the ability to taste PTC or not,
may involve some other gene interactions.
There’s even different ranges for how bitter
the chemical may taste because there may be
more kinds of alleles than we’ll mention---more
about that later.
But since we do know that the ability to taste
PTC or not taste PTC is at least heavily impacted
by a specific gene, it does make it powerful
for genetic classes.
One thing I found so interesting is that my
parents can both taste PTC.
So why can’t I?
Recall that humans have 46 chromosomes.
Chromosomes are made up of DNA and protein.
It’s a condensed unit of DNA.
My whole genetic code is represented by these
chromosomes.
You inherit 23 chromosomes from your mother
and 23 chromosomes from your father.
Here's all 46 of them right here. As you can see, there are 23 chromosome pairs.
Each pair has one chromosome from one parent
and one chromosome from my other parent.
If we focus on one of these pairs of chromosomes
where the PTC taste sensitivity gene may be
found, we can see an area where the PTC taste
sensitivity gene could be.
Let’s assume this is the locus where the
PTC taste sensitivity gene is found---see
how it is pointing to a specific area here?
That’s because it’s on an area on the chromosomes
that refers to a specific gene that codes
for a trait.
Now, remember how this chromosome is from
mom.
This one is from dad.
Each parent contributes an allele---which
is a variant of a gene.
An allele is a variety of a gene; a form of
a gene.
The alleles could be the same form of the
gene or different forms of the gene---but
regardless, in this case, they’re forms
of the gene involved with PTC taste sensitivity.
So if PTC taste sensitivity is being used
as a one gene trait example---and as we mentioned
it may not be that be simple---- then your
DNA code has a gene related to PTC taste sensitivity.
Together the two alleles you inherit, the
forms of that gene, determine the trait of
tasting PTC or the trait of not tasting PTC.
That gene is involved with coding for taste
receptors on your tongue and the receptors
you have can make a difference for whether
you taste PTC or not.
The alleles are typically represented by letters.
Since this is all about tasting, let’s use
the letter T. But wait---it matters whether
I represent it as a capital or lowercase letter!
If I use a capital letter to represent an
allele, it means it’s a dominant allele.
If one---or both---of the alleles you inherited
for a trait are dominant, then it will be
expressed.
More about that later.
If I use a lowercase letter to represent an
allele, that means it’s a recessive allele.
Recessive alleles are typically not expressed
unless there is no dominant allele present.
Now remember that you have two allele copies,
so the combinations you can have here could be TT, Tt, or
tt.
These are called genotypes.
Your genetic makeup.
Genotypes can help determine a phenotype,
which is a physical characteristic.
You’ll notice when writing genotypes, I
put the capital letters first if it contains
a capital letter.
That’s not because the order matters; it’s
a formatting formality that capitals are written
first.
It turns out that being able to taste PTC
is a dominant trait.
That means the phenotype, which is a
PTC taster, is due to a genotype that includes
at least one dominant allele.
So which genotypes can taste PTC then?
Well TT can; both of those alleles are dominant.
So can Tt, because remember it only takes
the presence of one dominant allele.
In fact, the only genotype in this simplified
example to not be able to taste PTC would
be tt.
So obviously that is what I am.
I am the tt genotype which results in my non-taster
phenotype.
But my parents can taste PTC...
So what genotypes would they have to be?
Well if they were both TT, that wouldn’t
be possible.
If one was TT and one was Tt, that still wouldn’t
be possible.
Remember you have to get an allele, a form
of a gene, from EACH parent.
If my parents do taste PTC and I do not, then
my parents have the genotype Tt.
And their phenotype is PTC taster.
Punnett squares can be used to determine the
probabilities of offspring having certain
genotypes---which then can be used to determine
their phenotypes.
But Punnett squares are for another Amoeba
Sisters video.
Before we end, one more thing to mention.
In this example, the dominant trait of being
able to taste PTC is more common than the
recessive trait of not being able to taste
PTC.
And one could jump to an assumption that dominant
traits are more common, especially since it
only takes the presence of one dominant allele
to show up in the phenotype.
At least, in Mendelian inheritance.
But the dominant trait is not always more
common in a population, because it's possible
that the dominant allele itself is more rare.
That can be the case with some forms of polydactyly…that is being born with extra fingers.
Some forms of polydactyly can be a dominant
trait caused by the presence of at least one
dominant allele; however, the dominant allele
may not be as common in the population and
the condition of having extra fingers is generally
rare.
Well that’s it for the amoeba sisters and
we remind you to stay curious.

Contents

Etymology

The word "allele" is a short form of allelomorph ("other form", a word coined by British geneticists William Bateson and Edith Rebecca Saunders),[4][5] which was used in the early days of genetics to describe variant forms of a gene detected as different phenotypes. It derives from the Greek prefix ἀλληλο-, allelo-, meaning "mutual", "reciprocal", or "each other", which itself is related to the Greek adjective ἄλλος, allos (cognate with Latinalius), meaning "other".

Alleles that lead to dominant or recessive phenotypes

In many cases, genotypic interactions between the two alleles at a locus can be described as dominant or recessive, according to which of the two homozygous phenotypes the heterozygote most resembles. Where the heterozygote is indistinguishable from one of the homozygotes, the allele expressed is the one that leads to the "dominant" phenotype,[6] and the other allele is said to be "recessive". The degree and pattern of dominance varies among loci. This type of interaction was first formally described by Gregor Mendel. However, many traits defy this simple categorization and the phenotypes are modeled by co-dominance and polygenic inheritance.

The term "wild type" allele is sometimes used to describe an allele that is thought to contribute to the typical phenotypic character as seen in "wild" populations of organisms, such as fruit flies (Drosophila melanogaster). Such a "wild type" allele was historically regarded as leading to a dominant (overpowering - always expressed), common, and normal phenotype, in contrast to "mutant" alleles that lead to recessive, rare, and frequently deleterious phenotypes. It was formerly thought that most individuals were homozygous for the "wild type" allele at most gene loci, and that any alternative "mutant" allele was found in homozygous form in a small minority of "affected" individuals, often as genetic diseases, and more frequently in heterozygous form in "carriers" for the mutant allele. It is now appreciated that most or all gene loci are highly polymorphic, with multiple alleles, whose frequencies vary from population to population, and that a great deal of genetic variation is hidden in the form of alleles that do not produce obvious phenotypic differences.

In the ABO blood group system, a person with Type A blood displays A-antigens and may have a genotype IAIA or IAi. A person with Type B blood displays B-antigens and may have the genotype IBIB or IBi. A person with Type AB blood displays both A- and B-antigens and has the genotype IAIB and a person with Type O blood, displaying neither antigen, has the genotype ii.

A population or species of organisms typically includes multiple alleles at each locus among various individuals. Allelic variation at a locus is measurable as the number of alleles (polymorphism) present, or the proportion of heterozygotes in the population. A null allele is a gene variant that lacks the gene's normal function because it either is not expressed, or the expressed protein is inactive.

For example, at the gene locus for the ABOblood typecarbohydrateantigens in humans,[7] classical genetics recognizes three alleles, IA, IB, and i, which determine compatibility of blood transfusions. Any individual has one of six possible genotypes (IAIA, IAi, IBIB, IBi, IAIB, and ii) which produce one of four possible phenotypes: "Type A" (produced by IAIA homozygous and IAi heterozygous genotypes), "Type B" (produced by IBIB homozygous and IBi heterozygous genotypes), "Type AB" produced by IAIB heterozygous genotype, and "Type O" produced by ii homozygous genotype. (It is now known that each of the A, B, and O alleles is actually a class of multiple alleles with different DNA sequences that produce proteins with identical properties: more than 70 alleles are known at the ABO locus.[8] Hence an individual with "Type A" blood may be an AO heterozygote, an AA homozygote, or an AA heterozygote with two different "A" alleles.)

Genotype frequencies

The frequency of alleles in a diploid population can be used to predict the frequencies of the corresponding genotypes (see Hardy-Weinberg principle). For a simple model, with two alleles;

p+q=1{\displaystyle p+q=1\,}

p2+2pq+q2=1{\displaystyle p^{2}+2pq+q^{2}=1\,}

where p is the frequency of one allele and q is the frequency of the alternative allele, which necessarily sum to unity. Then, p2 is the fraction of the population homozygous for the first allele, 2pq is the fraction of heterozygotes, and q2 is the fraction homozygous for the alternative allele. If the first allele is dominant to the second then the fraction of the population that will show the dominant phenotype is p2 + 2pq, and the fraction with the recessive phenotype is q2.

Other disorders, such as Huntington disease, occur when an individual inherits only one dominant allele.

Epialleles

While heritable traits are typically studied in terms of genetic alleles, epigenetic marks such as DNA methylation can be inherited at specific genomic regions in certain species, a process termed transgenerational epigenetic inheritance. The term epiallele is used to distinguish these heritable marks from traditional alleles, which are defined by nucleotide sequence.[9] A specific class of epiallele, the metastable epialleles, has been discovered in mice and in humans which is characterized by stochastic (probabilistic) establishment of epigenetic state that can be mitotically inherited.[10][11]